An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkynecis-Semihydrogenation with Ethanol: From Elementary Steps to Catalysis

Abstract

The selective synthesis of Z-alkenes in alkyne semihydrogenation relies on the reactivity difference of the catalysts toward the starting materials and the products. Here we report Z-selective semihydrogenation of alkynes with ethanol via a coordination-induced ionic monohydride mechanism. The EtOH-coordination-driven Cl– dissociation in a pincer Ir­(III) hydridochloride complex (NCP)­IrHCl (1) forms a cationic monohydride, [(NCP)­IrH­(EtOH)]+Cl–, that reacts selectively with alkynes over the corresponding Z-alkenes, thereby overcoming competing thermodynamically dominant alkene Z–E isomerization and overreduction. The challenge for establishing a catalytic cycle, however, lies in the alcoholysis step; the reaction of the alkyne insertion product (NCP)­IrCl­(vinyl) with EtOH does occur, but very slowly. Surprisingly, the alcoholysis does not proceed via direct protonolysis of the Ir–C­(vinyl) bond. Instead, mechanistic data are consistent with an anion-involved alcoholysis pathway involving ionization of (NCP)­IrCl­(vinyl) via EtOH-for-Cl substitution and reversible protonation of Cl– ion with an Ir­(III)-bound EtOH, followed by β-H elimination of the ethoxy ligand and C­(vinyl)–H reductive elimination. The use of an amine is key to the monohydride mechanism by promoting the alcoholysis. The 1–amine–EtOH catalytic system exhibits an unprecedented level of substrate scope, generality, and compatibility, as demonstrated by Z-selective reduction of all alkyne classes, including challenging enynes and complex polyfunctionalized molecules. Comparison with a cationic monohydride complex bearing a noncoordinating BArF– ion elucidates the beneficial role of the Cl– ion in controlling the stereoselectivity, and comparison between 1–amine–EtOH and 1–NaOtBu–EtOH underscores the fact that this base variable, albeit in catalytic amounts, leads to different mechanisms and consequently different stereoselectivity.

Pincer Iron Hydride Complexes for Alkene Isomerization: Catalytic Approach to Trisubstituted (Z)-Alkenyl Boronates

“…The stereoselectivity for the Z - over E -product can in part be attributed to the steric effect on the elimination step. The presence of a large Cl ligand in B versus the H ligand in B′ is in part responsible for the higher stereoselectivity in the isomerization of sterically unhindered substrates (e.g., 1a ) . In the case of hindered alkenes with bulky substituents (R) at the α-position, the steric repulsion between the R and Bpin groups in intermediate B′ is sufficient to induce high ( Z )-stereoselectivity.…”

Enantioselective α-Arylation of Primary Alcohols under Sequential One-Pot Catalysis

“…Similarly, no reaction was observed when Ru-3, an analogue of Ru-1 bearing acetate ligands in place of chloride anions, was used; but the reactivity of Ru-3 was partially recovered in the presence of additives bearing chloride anions (entries 8-9). These observations suggest that a mixed-ligand Ru-complex might be the active species; 30,31 however, further studies are required to verify this hypothesis. N-Ethylmaleimide HA-1 proved to be by far the most efficient as a sacrificial hydrogen acceptor among tested.…”

Manganese-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes with iPrOH as hydrogen source

“…The stereoselective semihydrogenation of alkynes to both Z - and E -olefins is relevant since these are valuable building blocks, with defined configurations, for the synthesis of organic compounds with academic and industrial applications. 3 Hence, many stoichiometric and catalytic systems have been reported for this type of transformations, with most of them usually based on late transition metals such as Ir, 4 Rh, 5 Ru, 6 and Pd. 7 Despite the high activity of noble metals in hydrogenations, due to matters of economy, sustainability and toxicity, efforts have been made in the last decade to develop catalytic processes based on first-row transition metals ( e.g.…”